The present invention relates to reducing "bleed" from organic-based formulations and materials useful in semiconductor assemblies. More particularly, the present invention relates to reducing resin bleed from organic-based materials such as die attach adhesives used to bond semiconductor devices to a substrate, including such materials as used in connection with PC boards, multi-chip modules, and in controlled collapse bump technology. The present invention encompasses compounds that reduce bleed when added to organic-based formulations as well as a method for determining such compounds.
Organic-based materials, such as die attach polymeric materials, have gained increasing popularity, in part for their ability to accommodate larger die sizes and to facilitate processing at lower temperatures. However, problems with bonded semiconductor devices resulting from "bleedout", also referred to as "bleed" or "resin bleed", or separation of the resin vehicle from the polymer adhesive, sometimes occur from the monomer as well as any other small molecule present. Such problems are discussed in detail in an article entitled "An Experimental Study of Die Attach Polymer Bleedout in Ceramic Packages" by Michael R. Marks, et al., which was published in Thin Solid Films, vol. 252, p. 54-60 in 1994. As indicated in the article, in severe bleedout conditions, the resin wets the entire substrate to which the die is to be attached and may succeed in "wicking up" the ceramic sidewalls and contaminate wire bonding metalization. This may result in weak wire bondings by virtue of acting as an interdiffusional barrier at the joints. Consequently, producers of the packages typically desire little or no bleed, or controlled bleed with tight tolerances.
The article by Marks et al. suggests two methods to eliminate bleedout: surface energy reduction of the gold plating (on the substrate) and improvement in the cohesiveness of the polymer die attach material. The author suggests that bleedout may be eliminated by increasing the cohesiveness of the polymer matrix by increasing the intermolecular attractive energy through the presence of functional groups with high dipole moments in high concentrations. In particular, Marks et al. suggest that phenol-cured epoxy, polyamide and polycyanate were found to have good bleedout resistance, which was attributed to high dipole moments of epoxide and hydroxyl in phenol-cured epoxy.
Die attach adhesive formulations that may be rapidly cured have been disclosed in Nguyen, U.S. Pat. No. 5,150,195, issued Sep. 22, 1992, and Nguyen, U.S. Pat. No. 5,155,066, issued Oct. 13, 1992, the disclosures of which are expressly incorporated herein by reference. Rapidly curable adhesive formulations such as those disclosed in the aforementioned patents may contain a cyanate ester vehicle, curing catalysts such as alkylphenols, metal curing catalysts such as metal chelates, and fillers, which may be electrically and/or thermally conductive. Such cyanate ester-containing formulations are capable of being fully cured in less than five minutes at 200.degree. C., and often in about two minutes, or less, at 200.degree. C.
However, in addition to rapid curability, it is important for a die attach adhesive formulation to minimize bleed or "flash-off." These terms describe the excessive spreading of a resin on an attached substrate during curing, and in the case of a cyanate ester formulation, flashing-off of the monomer, which may result in the monomer being redeposited in the vicinity of the bonded area. The problems of bleed and flash-off are of special concern in continuous in-line fast-curing processes, i.e., continuous processes for the production of bonded semiconductor assemblies.
Theoretically, bleed is a predictable event using the spreading equation described by Marangoni in Tipographia dei fatelli Fusi, Pavia in 1865 and by W. F. Cooper and W. H. Nuttal in the Journal of Agricultural Sciences Vol. 7, p. 219 in 1915. Such equations predict that if the combined liquid and interfacial energies are higher than that of the substrate, spreading will not occur. However, in order to predict spreading, the surface energies of the liquid, the solid and the interface must be known.
As a practical matter, these quantities are difficult and time consuming to measure. Therefore, using energy effects as a predictive tool is burdensome, requiring extensive testing and calculation.
The present invention provides a method to reduce bleed or bleedout of the organic-based formulations used, for example, in semi-conductor devices, and to novel formulations exhibiting reduced resin bleed as well as a method of selecting compounds that may be added to such polymeric materials to reduce bleed.